This invention relates generally to the field of gas or combustion turbines, and specifically to an apparatus and method for combusting a low quality fuel in a gas turbine engine.
Gas turbines are commonly used for combusting fossil fuels in order to generate power in the form of the mechanical energy of a rotating shaft. The rotating shaft mechanical energy is then converted into electrical energy in an electrical power generator. Either gaseous or liquid fossil fuel may be combusted in a gas turbine. Known fuel gases include propane, hydrogen and methane-based fuels such as natural gas. Liquid fuels include kerosene, aviation fuel and diesel fuel. For optimal efficiency and reliability, the combustor of a gas turbine must be designed with particular attention given to the type of fuel or fuels to be burned, since each fuel has its own specific combustion characteristics, and such characteristics affect the operation, longevity and reliability of the gas turbine.
One variable to be considered when designing a combustor is the amount of heat released during the combustion of the fuel. Heating value Q is defined as the energy released during combustion of the fuel per unit volume of the fuel. FIG. 1 itemizes several fuel gases and lists the associated Q value in units of BTU per cubic foot. Another variable to be considered when designing a combustor is the flammability of the fuel. Each fuel will support the propagation of a flame at only a specific range of concentration in air. Above a rich flammability limit and below a lean flammability limit, the fuel will not support flame combustion. The flammability range is the fuel concentration range from the lean flammability limit to the rich flammability limit. FIG. 1 illustrates the ratio of flammability limits for the various fuel gases. The ratio of flammability limits is defined as the rich flammability limit divided by the lean flammability limit. One may appreciate that fuels with higher ratios of flammability limits may be able to tolerate greater fluctuations in combustion parameters with a lower chance of unanticipated flame-outs when burned in a gas turbine combustor.
Natural gas is the fuel of choice for most modern gas turbine applications. As can be seen in FIG. 1, natural gas has a relatively high heating value, 950 BTU/ft3, and a mid-range ratio of flammability limits, thereby making it a good candidate for combustion in a gas turbine engine. Fuels with low heat value, such as low or medium energy coal gas, are considered acceptable fuels for combustion in a gas turbine so long as their ratio of flammability limits is sufficiently high, for example, at least equal to 2. Fuels referred to herein as high quality fuels are fuels that exhibit a ratio of flammability limits of 2 or more, and that have a heat value Q of at least 100 BTU/SCF. Fuels with low heating value and a low ratio of flammability limits, such as the low energy natural gas and blast furnace gas illustrated in FIG. 1, are generally considered to be unusable as fuel for a gas turbine engine. Such fuels are referred to herein as low quality fuels, meaning that they exhibit a ratio of flammability limits of less than 2, or that they have a heat value Q of below 100 BTU/SCF. The relatively high price of natural gas stimulates an ongoing interest in the use of lower cost fuels, such as gas produced by the gasification of low-grade biomass, by coal gasification or by petroleum coke gasification. Many of these fuel gases contain large amounts of moisture, carbon dioxide or other non-combustible compounds. The use of low heating value and/or narrow flammability range fuel requires special attention to combustor design. Current industrial gas turbine technology can effectively fire fuel gases with heating values as low as about 120 BTU/SCF and with a ratio of flammability of at least 2.
It is known to combust two different types of fuel in a single gas turbine. U.S. Pat. No. 5,359,847 issued to Pillsbury, describes a dual fuel ultra-low NOx combustor capable of operating on either a gaseous or liquid fuel. This design is optimized to reduce emissions of the oxides of nitrogen while burning high quality fuels at very lean fuel/air ratios.
U.S. Pat. No. 6,201,029 issued to Waycuilis, describes a gas turbine designed to operate on a low heating value fuel, such as may be obtained from a coal seam, landfill or sewage treatment plant. This design includes provisions for the supply of a high heating value fuel, such as natural gas, to supplement the supply of low heating value fuel. The combustion process is ignited with the high heating value fuel, then progressively more low heating value fuel is added to the fuel mixture. The patent describes a combustor specifically designed for burning the low heating value fuel and having a flame zone and an oxidation zone. The stability of the flame in this combustor is of concern. In addition to the special combustor design, this patent describes the preheating of the low heating value fuel in order to widen its flammability envelope, thereby providing additional stability to the combustion process.
It is known to combust low grade fuels and high grade fuels in separate combustion devices of a single power plant, as described in U.S. Pat. No. 5,934,065 issued to Bronicki, et al. Low grade fuels can be effectively combusted in a traditional burner due to the heat inertia of the burner walls, which are typically fire brick or other such material that is heated to an elevated temperature by the combustion process. The presence of such a radiant heat source provides stability for the flame. The approach of Bronicki requires combustor equipment that is separate from the gas turbine combustor, thereby increasing the capital cost of such a plant.
It is also known to utilize optical flame control to control the mixture of fuel and oxidizer in a burner, as described in U.S. Pat. No. 6,244,857 issued to VonDrasek, et al. The device of VonDrasek is used to control the relative flow rates of fuel and oxidizer in order to control a flame in a burner to be near stoichiometric conditions in order to limit the production of harmful emissions. The VonDrasek patent is not concerned with flame stability due to the thermal inertia available in the industrial burners of this application.
U.S. Pat. No. 5,978,525 issued to Shu, et al., describes a fiber optic sensor for detecting flashback occurrences in a combustor of a gas turbine. The device of Shu is limited to detecting the presence of a flame in a portion of the combustor where a flame is not normally present when the combustor is operating without flashback.
U.S. Pat. No. 6,024,561 issued to Kemp, et al., describes a flame detector for a burner that is used to alarm the absence of a flame, and also may be used to provide an input for the control of the air and fuel mixture flowing to the burner to achieve different output levels for the burner. The Kemp invention is not directed to the stability of the flame, nor is it directed to the combustion of low quality fuels.
A method of combusting fuel is described herein as including the steps of: providing a supply of a first fuel exhibiting a first quality parameter; providing a supply of a second fuel exhibiting a second quality parameter; combusting simultaneously in a combustor a flow of the first fuel and a flow of the second fuel; monitoring at least one parameter indicative of a level of stability of combustion in the combustor during the step of combusting; and controlling the flow of at least one of the first fuel and the second fuel in response to the at least one parameter. The parameter monitored may be electromagnetic radiation, pressure, or ionization potential in the combustor. The method may be applied when the first fuel exhibits a higher heat value than the second fuel, or when the first fuel exhibits a higher ratio of flammability limits than the second fuel. The method may further include increasing the relative flow rate of the first fuel compared to that of the second fuel when the step of monitoring indicates that combustion in the combustor is unstable.
An apparatus for combusting fuel is described herein as including: a combustion chamber; a compressor for supplying to the combustion chamber a flow of compressed air; a means for supplying to the combustion chamber a flow of a first fuel exhibiting a first quality parameter; a means for supplying to the combustion chamber a flow of a second fuel exhibiting a second quality parameter; a means for monitoring a level of stability of combustion in the combustion chamber; and a means for regulating at least one of the flow of compressed air, the flow of the first fuel and the flow of the second fuel in response to an output from the means for monitoring. The apparatus is further described as including: a sensor for providing a sensor signal responsive to the level of stability of combustion in the combustion chamber; a controller having the sensor signal as input and having an output; a device responsive to the controller output for regulating at least one of the flow of compressed air, the flow of the first fuel and the flow of the second fuel. The sensor may be an electromagnetic energy sensor, a pressure sensor, or a conductivity sensor.